This invention relates to gas discharge display panels. More particularly, it relates to a current level control for maintaining uniform brightness in each character display row in the panel.
In the referenced patent application Ser. No. 630,218 there is disclosed and claimed an MOS integrated circuit chip for both addressing and driving display panels. The MOS IC chip of that invention is particularly useful in driving the SELF-SCAN.TM. panel display which is sold commercially by Burroughs Corporation. The SELF-SCAN display panel includes a matrix of individual display cells filled with a gas which will provide a glow upon ionization. The display panel includes several character rows for displaying alphanumeric characters. Each character row is comprised of seven rows of individual cells or display devices. Each character row includes a plurality of crossing electrodes for supplying energization voltage and current to selected number of cells to ionize the gas therein and cause a glow discharge.
The MOS IC chip of the above identified patent application, is designed to drive one character display row. Since there are several character display rows in the SELF-SCAN display panel, a corresponding number of chips are required. The chips are identical in circuit and functional design. Unfortunately, however, the characteristics of each chip may be slightly different if they are fabricated at different times since it is impossible to maintain exact processing conditions during manufacture. For example, substrate material, thickness of the gate oxide, and mobility of the carriers may vary from one fabrication batch to another. These characteristics affect the voltage-current relation of each chip. Therefore, it is possible that the brightness of each character row in the display panel may be nonuniform since the characteristics of the chip driving the rows may be different.